At what point does the constructor of an injected service in Angular execute?

When using @Injectable({providedIn: 'root'}) in Angular 7 for a service, the constructor of the service executes when exactly? Is it upon the creation of a component that utilizes it as a dependency or does it wait until a method within the service is called first?

Note: Given that the service acts as a singleton, I am currently utilizing the constructor of the service to initialize certain values. Although an alternative approach could involve creating an "Initialize()" method and invoking it within the component's constructor, I personally find this method somewhat messy.

angulartypescriptangular7

Answer №1

Upon initialization of a class/service, the constructor that is decorated with the Injectable decorator gets called by the injector specific to its scope. It's not possible to have multiple instances of the same service within one scope unless explicitly specified using 

{ provide: MyService, useClass: MyService, multi: true }
.

The provideIn parameter determines the scope of the service, with provideIn: 'root' instructing the DI to use the RootInjector for injection. As a service is initialized only once within a scope, its constructor is invoked just once when the DI injects it into another component/service/module for the first time.

Answer №2

When a component or service needs an injected service, the instances of these services are created either when the requiring component/service is first created or when another service that requires them is instantiated.

Angular scans the Dependency Injection tree to find a definition and an instance of the required injected service when a component (or another service) is created or instantiated. If no instance is found, Angular creates one before running its constructor.

To ensure a service is instantiated before it is required by other objects/services, it can be added as a dependency in the APP_INITIALIZER array like this:

{ provide: APP_INITIALIZER, useFactory: () => () => null, deps: [MyService], multi: true }

This will initialize an instance of MyService when the app starts up, regardless of when other components/services need it. For more information, check out this link.

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